This invention relates to positive displacement rotary reciprocating piston machines of the type where the displacement of a piston within a cylinder causes fluid to be displaced within that cylinder.
For purposes of definition, a hydrostatic piston machine of the radial piston variety can either be of the type where a shaft-driven cylinder-barrel is mounted for rotation on a ported pintle-valve as disclosed in Ferris U.S. Pat. No. 2,105,454 or where the cylinder barrel is mounted for rotation on a revolving shaft. In the second type, a stationary axial distributor face valve is used in place of the pintle-valve and where a pair of kidney-shaped channels are used to fluidly connect with the cylinder-barrel to act as the means for porting the individual cylinders in the manner as shown in Tomell) U.S. Pat. No. 3,010,405.
In the first type of radial piston machine employing a pintle-valve, the cylinder-barrel is mounted for rotation about the longitudinal axis of the pintle-valve, and where the cylinder-barrel is provided with a series of cylinders. Each cylinder contains a piston and each piston is operatively connected to a surrounding annular track-ring. When the track-ring is positioned eccentric with respect to the cylinder-barrel, reciprocation of the pistons within their cylinders occurs. The arcuate-slots provided on the periphery of the pintle-valve and arranged to communicate through a series of fluid-passages which connect with fluid inlet and outlet conduits attached to the exterior of the housing of the machine. In the case of a hydraulic pump, rotary movement of the cylinder-barrel when the surrounding track-ring is eccentrically positioned, causes radial displacement of the pistons and a corresponding displacement of fluid from the "low-pressure" inlet conduit to the "higher-pressure" outlet conduit. The control-system of the machine determines the amount of track-ring eccentricity required in order that the resulting piston stroke is sufficient to meet the demands of a hydraulic system or circuit which the machine serves. In the case of an axial distributor face valve type of radial piston machine, kidney-shaped channels are used in place of such arcuate-slots, and where such kidney-shaped channels are formed on the axial distributor face valve which are arranged to fluidly connect with the cylinders provided in the rotatable cylinder-barrel.
Pintle-valves are thus well known and have been used in the art of radial piston machines for many decades. However, one constraint of using a pintle-valve is that only having a relatively small space is available within the pintle-valve for the inclusion of the necessary fluid-passages in order not to compromise the mechanical strength of the pintle-valve. In modern designs, this can be a problem as the pintle-valve is loaded in cantilever fashion by the radial forces emanating from the pressurized pistons. Therefore, such pintle-valve machines require careful application, especially when operated under certain conditions such as high-speed and in cold environments, in order to minimize the possibility of cavitation occurring in the relatively small fluid-passages in the pintle-valve. As a result, it is common practice to boost the inlet of such pintle-valve machines, usually by means of using a separate charge or boost pump.
A prior attempt for minimising the chances of cavitation occurring in the relatively small fluid-passages in the pintle-valve is shown in Great Britain Patent No. 524,384. In this pump, the fluid entering the space surrounding the rotating elements is propelled radially outwardly by centrifugal action, the centrifugally impelled fluid being arranged to pass through a diffuser passage provided in the track-ring from where it is piped to the low-pressure fluid passage in the pintle-valve. The disadvantage, however, is that the diffuser passage in the track-ring substantially weakens the strength of the track-ring. As a result, this pump is only suitable for relatively low-pressure applications or limited in the sense that the diameter of the pistons must be relatively small in size in order to avoid the track-ring becoming subjected to loads that could either cause the track-ring to deform or break due to the inherent weakness caused by the necessary addition of this diffuser passage. A new solution is therefore needed that does not compromise the strength of the track-ring or limit the unit to relatively low-pressure working applications.
It is also known practice to extend the drive-shaft of the radial piston machine axially in order that a further and separate hydraulic machine may be driven in the so-called "tandem" or "back-back" fashion. An example is shown in British Patent No. 1,465,876. This drive connection which passes through the centre of the radial piston machine, here called the first hydrostatic machine, maybe either in the form of a longitudinal extension to the drive-shaft or where a separate quill shaft is used in combination with the drive-shaft. In either case, for the purposes of further explanation, this drive connection will be referred to as a through-shaft. The need to include such a through-shaft for driving a second hydrostatic machine causes further difficulties because the low pressure fluid-passage in the pintle-valve of the first radial piston machine has to be restricted in size to allow space for the inclusion of a central longitudinal aperture in the pintle-valve in which the through-shaft passes. As the diameter of the pintle-valve is determined by various design parameters such as the generated area for the hydrostatic bearing field for enabling the piston loads to be supported, it is normally not possible to just exaggerate the size of the pintle-valve to provide more internal space for the fluid-passages and aperture. As such, the additional space required within the pintle-valve for the aperture through which the through-shaft passes means that the cross-sectional area of the low-pressure fluid-passage has to be arranged even smaller then would be normally the case if the requirement to drive a second hydrostatic machine were not needed. Having therefore to reduce the size of the suction or low-pressure fluid-passage in the pintle-valve in order to meet the requirement to drive the second hydrostatic machine accordingly may fisher increase the chances of cavitation occurring.
The addition of a separate "boost" pump, here called the third hydrostatic machine, into the circuit that acts to keep the first hydrostatic machine fully "primed" with fluid regardless of its operating conditions is the current practice, but this is not only costly to perform but further complicates matters because most often, the ideal location to mount and drive such a "boost" pump to the back of the second hydrostatic machine. Having to include a sufficiently large through-shaft in the first hydrostatic machine that can carry the driving torque required by both the second and third hydrostatic machines means in practice, that the space available within the pintle-valve for the fluid-passages is further reduced. Unless the through-shaft used is sufficiently large, is unlikely to have the required strength to be able to transmit the full driving torque that the second and third machines may demand on occasion. There therefore is a problem with current radial piston machines design employing pintle-valves.
By contrast, the size of fluid-passages used in an axial distributor face valve type of radial piston machine are not restricted in size even when a relatively large through-shaft is needed for driving further hydrostatic machines because the radial location of such passages and their corresponding kidney-shaped channels is at a greater pitch circle diameter than is possible with the pintle-valve type of radial piston machine. However, during cold weather operations at high speeds, cavitation may still occur, because the fluid passages in the cylinder-barrel that connect the cylinders to the kidney-shaped channels in the axial distributor face valve are rather small in cross-sectional area. Consequently, a separate "boost" pump may be required to boost the inlet of the first hydrostatic machine There therefore is also a need to provide an improved fluid circuit for the axial distributor face valve type of radial piston machine in order to eliminate the need of having to fit a third hydrostatic machine for such boosting purposes.
A further problem exists when the first hydrostatic machine, irrespective of whether it is uses a pintle-valve or axial distributor face valve, is driven for long periods at zero or minimal fluid output. Under such operating conditions, the heat generated inside the machine from hydro-mechanical losses may not be expelled sufficiently quickly into the connecting hydraulic fluid circuit. Overheating causes elements such as the seals to fail reducing the useful working life of the hydrostatic machine.
There is therefore a need in the art for a new radial piston hydrostatic machine that overcomes these known disadvantages of the prior art types.